Lesson starts are transitional events which may cause management problems for teachers. In this study 131 lesson starts of equally many teachers were observed in primary and secondary schools in Finland. The results indicated that, in general, the problems were minimal. However, for various reasons lesson starts were delayed by an average of about six minutes. Calculated on this basis, the total loss of instructional time in the whole school year was about five weeks of schooling. No statistically significant relationships were observed between disturbances in the classroom and any background variable studied including grade level, classroom type, (special or mainstream), group size, presence of classroom assistant, sex of the teacher, weekday, time of day of the lesson, or subject of the lesson. In order to maximise instructional time more attention should be paid in future to starting lessons promptly.